1. Field of the Invention
This invention relates to a sheet conveyor system which conveys a plurality of kinds of sheet material different in width, and more particularly to such a sheet conveyor system which can convey straight the sheet to be conveyed irrespective of width.
2. Description of the Related Art
Recently, there has been put into practice a system in which a radiation image of an object such as a human body is once stored on a stimulable phosphor sheet (or a radiation image conversion panel) by exposing the stimulable phosphor sheet to radiation through the object to have the stimulable phosphor sheet store radiation energy, and stimulated emission which is emitted from each part of the stimulable phosphor sheet upon exposure to stimulating light in proportion to the radiation energy stored thereon is photoelectrically read, thereby obtaining a digital image signal representing the radiation image stored on the stimulable phosphor sheet, and the digital image signal is reproduced as a visible image on a recording medium such as a photographic film or on an image display system such as those using a CRT.
In such a system, the stimulable phosphor sheets are handled with each contained in one cassette or with the sheets contained in one magazine by two or more.
In the system, there is generally employed a radiation image information read-out apparatus provided with a read-out section for reading out a radiation image stored in stimulable phosphor sheets and an erasing section for exposing the stimulable phosphor sheet to erasing light after the image signal is obtained from the stimulable phosphor sheet so that the residual energy of the radiation is fully released from the stimulable phosphor sheet. In the radiation image information read-out apparatus, stimulable phosphor sheets on which radiation images of objects have been recorded by external radiation image recording apparatus are loaded with each stimulable phosphor sheet contained in a cassette or with the sheets contained in magazines by two or more. (The cassettes and the magazines will be referred to as a xe2x80x9ccontainerxe2x80x9d, hereinbelow.) Then the lid of the container is opened and a sheet conveyor system takes out the stimulable phosphor sheets from the container one by one and conveys the stimulable phosphor sheet to the read-out section.
The read-out section reads a radiation image recorded on the stimulable phosphor sheet. After read-out, the stimulable phosphor sheet is transferred to the erasing section and the residual energy of the radiation is fully released from the stimulable phosphor sheet. Thereafter, the renewed stimulable phosphor sheet is returned to the original container or put in another container and removed from the radiation image information read-out apparatus together with the container.
Further, the aforesaid system is provided with an image reproducing system for reproducing a radiation image on a photosensitive material such as a photographic film. In the image reproducing system, a plurality of sheets of photosensitive material are taken out from a magazine one sheet by one sheet by a suction mechanism and transferred to a sheet conveyor system, which conveys the photosensitive material sheet to a recording system. The recording system records a radiation image on the photosensitive material sheet, for instance, by exposing the sheet to a laser beam on the basis of an image signal obtained from the stimulable phosphor sheet.
The stimulable phosphor sheet and the photographic film (such sheets will be simply referred to as a xe2x80x9csheetxe2x80x9d, hereinbelow) are in various sizes by purpose. Accordingly, the sheet conveyor system generally conveys the sheet with one side edge of the sheet kept in a reference position.
FIG. 10 shows an example of the conventional sheet conveyor system. As shown in FIG. 10, the conventional sheet conveyor system has a roller pair 103, one of which is a drive roller 101 which is driven by, for instance, an electric motor (not shown) and the other of which is a nip roller 102 which is positioned above the drive roller 101 and is associated with the drive roller 101 to nip therebetween a sheet 140 to be conveyed. The drive roller 101 comprises a shaft 111 and three roller portions 112A, 112B and 112C which are of rubber and mounted on the shaft 111 at predetermined intervals. Similarly the nip roller 102 comprises a shaft 121 and three roller portions 122A, 122B and 122C which are of rubber and mounted on the shaft 121 at predetermined intervals. The nip roller 102 is pressed against the drive roller 101 by compression springs 127a and 127b by way of bearings 125a and 125b disposed on opposite ends thereof. When the drive roller 101 is driven, the sheet 140 is conveyed nipped between the drive roller 101 and the nip roller 102 with one side edge 140a of the sheet 140 held in a reference position 105 irrespective of the width of the sheet 140. That is, when the width of the sheet 140 is small, the sheet 140 is conveyed nipped between the roller portions 112A and 122A and 112B and 122B at its opposite edge portions. When the width of the sheet 140 is large, the sheet 140 is conveyed nipped between the roller portions 112A and 122A, 112B and 122B and 112C and 122C at its opposite edge portions and an intermediate portion.
Setting of force of the springs 127a and 127b will be described, hereinbelow. When a large size sheet 140 is to be conveyed as shown in FIG. 11, the conveying force is maximized at the outer side of the roller portions 112A and 122A and at the outer side of the roller portions 112C and 122C. Accordingly when the nipping force f1 acting at the outer side of the roller portions 112A and 122A is equal to the nipping force f2 acting at the outer side of the roller portions 112C and 122C, the sheet 140 can be conveyed straight. The nipping force f1 acting at the outer side of the roller portions 112A and 122A can be made equal to the nipping force f2 acting at the outer side of the roller portions 112C and 122C, when the force F1 of the compression spring 127a is set equal to the force F2 of the compression spring 127b. 
To the contrast, when a small size sheet 140 is to be conveyed, the sheet 140 is nipped only between the roller portions 112A and 122A and between the roller portions 112B and 122B and as a result the roller portions 112C and 122C are spaced from each other as shown in FIG. 10. When a space is formed between the roller portions 112C and 122C while the opposite ends of the nip roller are pressed against the drive roller 101 by the forces F1 and F2 of the compression springs 127a and 127b, a moment which tends to nullify the space acts on the nip roller 102 together with the gravity of the roller portion 122C, which makes the nip roller 102 inclined in a direction in which the space is nullified. When the nip roller 102 is thus inclined, the nipping force acting between the roller portions 112B and 122B becomes stronger than that acting between the roller portions 112A and 122A and the portion of the sheet 140 between the roller portions 112B and 122B comes to be conveyed at a higher speed than the portion of the sheet 140 between the roller portions 112A and 122A, whereby the sheet 140 comes to be conveyed obliquely rightward as seen in FIG. 10. Further, since the roller portions 112B and 122B are of rubber, the roller portions 112B and 122B are apt to collapse and accordingly, it is difficult to prevent production of a difference in conveying speed. In view of conveying straight small size sheets, the forces of the springs 127a and 127b should be set in the following manner.
When a small size sheet 140 is to be conveyed as shown in FIG. 12, the conveying force is maximized at the outer side of the roller portions 112A and 122A and at the outer side of the roller portions 112B and 122B. Accordingly when the nipping force f3 acting at the outer side of the roller portions 112A and 122A is equal to the nipping force f4 acting at the outer side of the roller portions 112B and 122B, the sheet 140 can be conveyed straight. The nipping force f3 acting at the outer side of the roller portions 112A and 122A can be made equal to the nipping force f4 acting at the outer side of the roller portions 112B and 122B, when F3xc2x7L=F4xc2x7L/(Lxe2x88x92L2)+gravity of the roller portion 122C, wherein L represents the distance between the fulcrums of opposite ends of the nip roller 102 (i.e., the distance between the bearings 125a and 125b, L2 represents the distance between the bearing 125b and the outer side of the roller portions 112B and 122B, and F3 and F4 respectively represent the forces of the compression springs 127a and 127b. That is, F3 greater than F4.
This means that the small size sheet 140 can be conveyed straight when the force F3 of the compression spring 127a on the side of the reference position 105 is stronger than the force F4 of the compression spring 127b so that the nipping force acting on the sheet 140 between the roller portions 112A and 122A becomes substantially equal to that acting on the sheet 140 between the roller portions 112B and 122B.
Whereas when the urging force of the compression spring 127a on the side of the reference position 105 is stronger than that of the compression spring 127b (F1 greater than F2, F3 greater than F4), the nipping force acting between the roller portions 112A and 122A becomes stronger than that acting between the roller portions 112C and 122C when the large size sheet 140 is conveyed and the portion of the sheet 140 between the roller portions 112A and 122A comes to be conveyed at a higher speed than the portion of the sheet 140 between the roller portions 112C and 122C, whereby the sheet 140 comes to be conveyed obliquely leftward, though the degree of inclination is suppressed by the friction force between the sheet 140 and the roller portions 112A, 112B, 112C, 122A 122B and 122C.
However, when the stimulable phosphor sheet is conveyed obliquely in the radiation image information read-out apparatus, the edges of the image obtained are inclined and the image becomes unsightly even if the inclination of the stimulable phosphor sheet is slight. Though inclination of the edges of the image can be nullified by image processing, it becomes impossible to nullify the inclination of the edges of the image when the degree of inclination exceeds a certain value. That is, it is required to convey the sheet as straight as possible.
In view of the foregoing observations and description, the primary object of the present invention is to provide a sheet conveyor system which can convey straight a sheet of any size.
In accordance with the present invention, there is provided a sheet conveyor system which conveys a plurality of kinds of sheet material, different at least in dimension in a first direction, in a second direction normal to the first direction with an edge of each sheet material extending in the second direction held along a reference position, the sheet conveyor system comprising
a roller pair which consists of a drive roller and a nip roller and conveys the sheet material in the second direction by driving the drive roller with the nip roller urged toward the drive roller under its gravity and a predetermined urging force to press the sheet material against the drive roller, wherein the improvement comprises that
the nip roller is larger in weight of a part on the side of the reference position than that of a part on the side opposite to the reference position and is substantially uniform in weight over a predetermined length between the end on the side of the reference position and a part at a predetermined distance from the end on the side of the reference position, and
said predetermined urging force is set stronger on the side opposite to the reference position than on the side of the reference position.
The predetermined length is set according to the dimension in the first direction of the sheet material which is the smallest in the dimension in the first direction in the sheet materials to be conveyed. For example, the predetermined length is set to a half of the dimension in the first direction of the sheet material which is the smallest in the dimension in the first direction in the sheet materials to be conveyed.
When the weight of the nip roller on the side of the reference position is larger than the weight of the part on the side opposite to the reference position and a sheet material whose width is substantially equal to the length of the nip roller is conveyed, the sheet nipping force becomes stronger on the reference position side and the sheet material is inclined away from the reference position. The inclination of the sheet material can be cancelled by setting said predetermined urging force stronger on the side opposite to the reference position than on the side of the reference position.
For example, each of the drive roller and the nip roller may comprise a shaft extending in the first direction (the direction of width of the sheet material) and a plurality of roller portions provided on the shaft at predetermined intervals in the longitudinal direction thereof, with the part between the roller portion nearest to the reference position and the roller portion adjacent to the roller portion nearest to the reference position being uniform in weight.
The predetermined interval is an interval such that the roller portions can nip the sheet material at its side edges or portions near to the side edges irrespective of the size of the sheet material. For example, when the sizes of the sheet materials to be conveyed are only a large size which is substantially equal to the length of the nip roller in dimension in the first direction and a small size which is substantially equal to a half of the length of the nip roller in dimension in the first direction, the predetermined interval is such that three roller portions are disposed respectively on opposite ends of the nip roller and the center of the same.
It is preferred that the roller portion be formed of a high friction material such as rubber. Further when the roller portion is 1.5 to 2 mm in thickness, collapse by the urging force of the nip roller can be suppressed.
The shaft of the nip roller may be larger in outer diameter at the reference position side portion than that at the portion remote from the reference position.
The shaft of the nip roller may comprise, for instance, a hollow pipe-like member and a core shaft which supports the pipe-like member for rotation with the core shaft being larger in outer diameter at the reference position side portion than that at the portion remote from the reference position.
Further, the reference position side portion of the nip roller may be formed of a material which is heavier than that forming the portion remote from the reference position.
In the sheet conveyor system in accordance with the present invention, the nip roller is urged toward the drive roller under its gravity and a predetermined urging force when conveying a sheet material. Since the nip roller is larger in weight of a part on the side of the reference position than that of a part on the side opposite to the reference position and is substantially uniform in weight over a predetermined length between the end on the side of the reference position and a part at a predetermined distance from the end on the side of the reference position, sheet materials which are small in width than the predetermined length can be conveyed substantially straight since the pressing force acting on the sheet material during conveyance is substantially uniform in the direction of width even if the urging force is stronger on the side opposite to the reference position than on the side of the reference position.
When the sheet material to be conveyed is substantially equal in width to the length of the nip roller, the sheet material can be conveyed substantially straight since the pressing force acting on the sheet material during conveyance is substantially equal at opposite edges of the sheet material since the urging force is stronger on the side opposite to the reference position than on the side of the reference position though the pressing force acting on the sheet material under the gravity of the nip roller is stronger on the reference position side than the side opposite to the reference position.